[PATCH net 2/2] selftests: net: packetdrill: test RACK after partial undo

[nramaswamy@openai.com]

From: Neil Ramaswamy <nramaswamy@openai.com>

Reproduces a partial undo bug where a segment is unmarked as lost but is
never returned to RACK's timestamp sorted list, which makes it ineligible
for future fast retransmission.

Signed-off-by: Neil Ramaswamy <nramaswamy@openai.com>
Assisted-by: LLM sparse
---
 ...tcp_partial_undo-restores-to-rack-list.pkt | 54 +++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt

diff --git a/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt b/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt
new file mode 100644
index 000000000000..c07a2f8a5cbd
--- /dev/null
+++ b/tools/testing/selftests/net/packetdrill/tcp_partial_undo-restores-to-rack-list.pkt
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0
+//
+// Test that a segment unmarked as lost during partial undo is eligible
+// for future fast retransmission.
+
+`./defaults.sh`
+
+// Establish a connection with a 100 ms RTT and a 1000-byte payload MSS.
+// Linux subtracts the 12-byte timestamp options from the advertised MSS.
+    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+   +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+   +0 bind(3, ..., ...) = 0
+   +0 listen(3, 1) = 0
+
+  +.1 < S 0:0(0) win 20000 <mss 1012,sackOK,TS val 1000 ecr 0>
+   +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 100 ecr 1000>
+  +.1 < . 1:1(0) ack 1 win 20000 <nop,nop,TS val 1100 ecr 100>
+   +0 accept(3, ..., ...) = 4
+
+// Send A, B, C and D, then E 47 ms after D.
+ +.01 write(4, ..., 1000) = 1000
+   +0 > P. 1:1001(1000) ack 1 <nop,nop,TS val 210 ecr 1100>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 1001:2001(1000) ack 1 <...>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 2001:3001(1000) ack 1 <...>
++.001 write(4, ..., 1000) = 1000
+   +0 > P. 3001:4001(1000) ack 1 <...>
++.047 write(4, ..., 1000) = 1000
+   +0 > P. 4001:5001(1000) ack 1 <...>
+
+// SACK C and E together. RACK marks A, B and D lost. D is old enough
+// to be retransmitted, but this ACK reports only two newly delivered
+// segments, allowing A and B to be retransmitted while D waits.
+ +.12 < . 1:1(0) ack 1 win 20000 <TS val 1280 ecr 100,sack 4001:5001 2001:3001>
+   +0 > P. 1:1001(1000) ack 1 <...>
+   +0 > P. 1001:2001(1000) ack 1 <...>
+   +0 %{
+assert tcpi_ca_state == TCP_CA_Recovery, tcpi_ca_state
+assert tcpi_lost == 3, tcpi_lost
+assert tcpi_retrans == 2, tcpi_retrans
+}%
+
+// Deliver retransmitted B and then the original A. D stays missing,
+// and we ACK through C with A's original timestamp, which is before
+// retransmission started. This triggers partial undo, but D should
+// remain eligible for fast retransmission (critically, the timeout
+// retransmission counter should be 0).
+ +.07 < . 1:1(0) ack 3001 win 20000 <TS val 1350 ecr 210,sack 4001:5001>
+ +0~+.05 > P. 3001:4001(1000) ack 1 <nop,nop,TS val 450 ecr 1350>
+   +0 %{ assert tcpi_retransmits == 0, tcpi_retransmits }%
+
+// Acknowledge all five segments.
+ +.01 < . 1:1(0) ack 5001 win 20000 <nop,nop,TS val 1360 ecr 450>